JP6002508B2 - Nitride semiconductor wafer - Google Patents

Nitride semiconductor wafer Download PDF

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JP6002508B2
JP6002508B2 JP2012193047A JP2012193047A JP6002508B2 JP 6002508 B2 JP6002508 B2 JP 6002508B2 JP 2012193047 A JP2012193047 A JP 2012193047A JP 2012193047 A JP2012193047 A JP 2012193047A JP 6002508 B2 JP6002508 B2 JP 6002508B2
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nitride semiconductor
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土屋　忠厳
忠厳 土屋
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Sumitomo Chemical Co Ltd
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Description

本発明は、窒化物半導体ウェハに関する。   The present invention relates to a nitride semiconductor wafer.

Ｓｉ、ＳｉＣ、ＧａＮ等からなる導電性あるいは半絶縁性の半導体基板の（１１１）面上、あるいはＣ面〔（０００１）面〕上に、高電子移動度トランジスタ（ＨＥＭＴ）などのヘテロ接合を有するデバイス構造を形成する場合、バッファ層に高い絶縁特性が求められる。しかし、単層のバッファ層を用いた場合は、バッファ層内にエネルギー障壁が無く、バッファ層に注入された自由電子が動きやすいため、必ずしも十分な絶縁特性が得られない。   A heterojunction such as a high electron mobility transistor (HEMT) is formed on the (111) plane or the C plane ((0001) plane) of a conductive or semi-insulating semiconductor substrate made of Si, SiC, GaN or the like. When forming a device structure, high insulation characteristics are required for the buffer layer. However, when a single buffer layer is used, there is no energy barrier in the buffer layer, and free electrons injected into the buffer layer easily move, so that sufficient insulation characteristics cannot always be obtained.

絶縁特性を向上させるためには、例えば、窒化アルミニウムガリウム（ＡｌＧａＮ）層と窒化ガリウム（ＧａＮ）層とのヘテロ接合を含むバッファ層を用いることが有効であるが、窒化物半導体特有の分極効果のために生じる誘起電荷が、絶縁特性向上の障害となる。ｃ軸方向に積層したＡｌＧａＮ／ＧａＮへテロ接合構造は、窒化物系半導体結晶に固有の自発分極及びピエゾ分極によって、界面に反対符号の自由キャリアが誘起されやすい。   In order to improve the insulation characteristics, for example, it is effective to use a buffer layer including a heterojunction of an aluminum gallium nitride (AlGaN) layer and a gallium nitride (GaN) layer. The induced charge generated for this is an obstacle to the improvement of the insulation characteristics. In the AlGaN / GaN heterojunction structure laminated in the c-axis direction, free carriers with opposite signs are likely to be induced at the interface due to the spontaneous polarization and piezoelectric polarization inherent in the nitride-based semiconductor crystal.

上記のようなヘテロ接合を含むバッファ層においては、分極モーメントがＡｌｘＧａ１−ｘＮ層のＡｌ組成ｘに依存するため、分極電荷密度はＡｌ組成ｘに依存する（例えば、非特許文献１参照）。非特許文献１によれば、ＡｌＧａＮ層中に固有に存在する自発分極とＧａＮ層の中に固有に存在する自発分極との差分に相当する電荷σＰｓｐ／ｅがＡｌＧａＮ層とＧａＮ層の界面に誘起される。さらに、外力に応じて発生するＡｌＧａＮ層中の弾性応力によるＡｌＧａＮ層中のピエゾ分極と外力に応じて発生するＧａＮ層中の弾性応力によるＧａＮ層中のピエゾ分極との差分に相当する電荷σＰｐｅ／ｅがＡｌＧａＮ層とＧａＮ層の界面に誘起される。このため、上記のσＰｓｐ／ｅとσＰｐｅ／ｅとの合計σ／ｅ（Ｐｓｐ＋Ｐｐｅ）がＡｌＧａＮ層とＧａＮ層の界面に誘起される全分極電荷に相当する。 In the buffer layer including the heterojunction as described above, since the polarization moment depends on the Al composition x of the Al x Ga 1-x N layer, the polarization charge density depends on the Al composition x (for example, Non-Patent Document 1). reference). According to Non-Patent Document 1, a charge σP sp / e corresponding to the difference between the spontaneous polarization inherent in the AlGaN layer and the spontaneous polarization inherent in the GaN layer is present at the interface between the AlGaN layer and the GaN layer. Induced. Further, a charge σP pe corresponding to the difference between the piezoelectric polarization in the AlGaN layer due to the elastic stress in the AlGaN layer generated according to the external force and the piezoelectric polarization in the GaN layer due to the elastic stress in the GaN layer generated according to the external force. / E is induced at the interface between the AlGaN layer and the GaN layer. For this reason, the total σ / e (P sp + P pe ) of the above σP sp / e and σP pe / e corresponds to the total polarization charge induced at the interface between the AlGaN layer and the GaN layer.

そして、ＡｌＧａＮ層の組成式ＡｌｘＧａ１−ｘＮのＡｌ組成ｘとσ／ｅ（Ｐｓｐ＋Ｐｐｅ）との関係を実験により求めると、この関係を表す曲線は、関係式σ／ｅ（Ｐｓｐ＋Ｐｐｅ）＝５×１０１３・ｘ（ｘ≦０．６）、σ／ｅ（Ｐｓｐ＋Ｐｐｅ）＝１×１０１４・ｘ−３×１０１３（ｘ＞０．６）で表される。 Then, when the relationship between the Al composition x of the AlGaN layer Al x Ga 1-x N and σ / e (P sp + P pe ) is obtained by experiments, the curve representing this relationship is expressed by the relationship σ / e ( P sp + P pe ) = 5 × 10 13 · x (x ≦ 0.6), σ / e (P sp + P pe ) = 1 × 10 14 · x−3 × 10 13 (x> 0.6) Is done.

すなわち、非特許文献１によれば、ＡｌＧａＮ／ＧａＮへテロ接合構造を有するバッファ層において、ＡｌＧａＮ層の組成式をＡｌｘＧａ１−ｘＮ、バッファ層中の各々のＡｌＧａＮ層とＧａＮ層の界面の分極電荷密度をＮｐとすると、ｘ≦０．６のときは、概ねＮｐ＝５×１０１３・ｘ（ｃｍ−２）で表され、ｘ＞０．６のときは、概ねＮｐ＝１×１０１４・ｘ−３×１０１３（ｃｍ−２）で表される。例えば、Ａｌ組成ｘが０．１である場合は、分極電荷密度Ｎｐは概ね５ｘ１０１２（ｃｍ−２）である。 That is, according to Non-Patent Document 1, in a buffer layer having an AlGaN / GaN heterojunction structure, the composition formula of the AlGaN layer is Al x Ga 1-x N, and the interface between each AlGaN layer and the GaN layer in the buffer layer When the polarization charge density of Np is Np, it is approximately represented by Np = 5 × 10 13 · x (cm −2 ) when x ≦ 0.6, and approximately Np = 1 × when x> 0.6. It is represented by 10 14 · x−3 × 10 13 (cm −2 ). For example, when the Al composition x is 0.1, the polarization charge density Np is approximately 5 × 10 12 (cm −2 ).

分極電荷は層の両端に反対符号の電荷を誘起するが、ＡｌＧａＮ／ＧａＮへテロ接合構造が独立系である場合は、電気的には中性であって問題はない。しかし、その上層にＨＥＭＴ構造などのアクティブ層が形成されると、それによってバンドダイアグラムが変化して、部分的に中性条件から外れる。自由電子をキャリアとするモノポーラデバイスであるＨＥＭＴエピ構造では、バッファ層のＡｌＧａＮ／ＧａＮ界面にも電子が優勢的に誘起されやすくなる。これを抑えるためには、バッファ層へのアクセプタの導入が有効であると考えられる。   Polarization charges induce charges of opposite signs at both ends of the layer, but when the AlGaN / GaN heterojunction structure is an independent system, it is electrically neutral and causes no problem. However, when an active layer such as a HEMT structure is formed on the upper layer, the band diagram is changed thereby, and partially deviates from the neutral condition. In the HEMT epi structure, which is a monopolar device using free electrons as carriers, electrons tend to be predominantly induced at the AlGaN / GaN interface of the buffer layer. In order to suppress this, it is considered effective to introduce an acceptor into the buffer layer.

従来、組成の異なる２種の窒化アルミニウムガリウム層の交互層からなる多層バッファ層に、炭素を含有させた化合物半導体基板が知られている（例えば、特許文献１参照）。特許文献１によれば、多層バッファ層が炭素を含有することにより、デバイスの耐電圧性能が向上する。   2. Description of the Related Art Conventionally, a compound semiconductor substrate in which carbon is contained in a multilayer buffer layer composed of alternating layers of two types of aluminum gallium nitride layers having different compositions is known (for example, see Patent Document 1). According to Patent Document 1, the withstand voltage performance of the device is improved when the multilayer buffer layer contains carbon.

特開２０１１−８２４９４号公報JP 2011-82494 A

O. Ambacherら，Journal of Applied Physics，Vol.87，334頁，2000O. Ambacher et al., Journal of Applied Physics, Vol.87, 334, 2000

しかし、特許文献１に記載された化合物半導体基板のように、多層バッファ層全体に炭素をドーピングすると、アクセプタとして機能する炭素がチャネル層の自由電子も捕獲してしまうおそれが高く、デバイスのオン抵抗が上昇し、デバイス特性が悪化してしまうという問題がある。   However, when carbon is doped in the entire multilayer buffer layer as in the compound semiconductor substrate described in Patent Document 1, there is a high possibility that carbon functioning as an acceptor captures free electrons in the channel layer, and the on-resistance of the device. As a result, there is a problem that device characteristics deteriorate.

したがって、本発明の目的の一つは、オン抵抗特性及びバッファリーク特性に優れるデバイスの製造に用いることができる、ヘテロ接合を含むバッファ層を有する窒化物半導体ウェハを提供することにある。   Accordingly, one object of the present invention is to provide a nitride semiconductor wafer having a buffer layer including a heterojunction, which can be used for manufacturing a device having excellent on-resistance characteristics and buffer leakage characteristics.

（１）また、本発明の他の態様によれば、上記目的を達成するため、基板と、ＡｌｘＧａ１−ｘＮ層（０≦ｘ≦０．０５）と、ＡｌｙＧａ１−ｙＮ層（０＜ｙ≦１、かつｘ＜ｙ）との交互層を含む、前記基板上のバッファ層と、を有し、前記交互層において、前記ＡｌｙＧａ１−ｙＮ層のみがアクセプタを含み、前記交互層の前記ＡｌｙＧａ１−ｙＮ層のうちの最上層のＡｌｙＧａ１−ｙＮ層のＡｌ組成ｙは、０＜ｙ≦０．４であり、前記最上層のＡｌｙＧａ１−ｙＮ層の前記アクセプタのシート濃度Ｎｓは、５×１０１３・ｙ＜Ｎｓ≦２×１０１３（ｃｍ−２）である、窒化物半導体ウェハが提供される。 (1) According to another aspect of the present invention, for achieving the above object, a substrate, Al x Ga 1-x N layer (0 ≦ x ≦ 0.05), Al y Ga 1-y A buffer layer on the substrate including alternating layers with N layers (0 <y ≦ 1 and x <y), wherein only the Al y Ga 1-y N layer is an acceptor. wherein the said Al y Ga 1-y N Al composition y of the uppermost Al y Ga 1-y N layer of layers of the alternating layer is 0 <y ≦ 0.4, the uppermost A nitride semiconductor wafer in which the acceptor sheet concentration Ns of the Al y Ga 1-y N layer is 5 × 10 13 · y <Ns ≦ 2 × 10 13 (cm −2 ) is provided.

（２）上記窒化物半導体ウェハにおいて、前記アクセプタは、鉄（Ｆｅ）、炭素（Ｃ）、亜鉛（Ｚｎ）のうちの少なくとも１種類からなることが好ましい。 ( 2 ) In the nitride semiconductor wafer, the acceptor is preferably made of at least one of iron (Fe), carbon (C), and zinc (Zn).

（３）上記窒化物半導体ウェハにおいて、前記ＡｌｘＧａ１−ｘＮ層のＡｌ組成ｘは、ｘ＝０であることが好ましい。 ( 3 ) In the nitride semiconductor wafer, the Al composition x of the Al x Ga 1-x N layer is preferably x = 0.

（４）上記窒化物半導体ウェハにおいて、前記交互層のすべての前記ＡｌｙＧａ１−ｙＮ層のＡｌ組成ｙ及び一層あたりの前記アクセプタのシート濃度Ｎｓが等しいことが好ましい。 ( 4 ) In the nitride semiconductor wafer, it is preferable that the Al composition y of all the Al y Ga 1-y N layers of the alternating layers and the sheet concentration Ns of the acceptor per layer are equal.

本発明の一態様によれば、オン抵抗特性及びバッファリーク特性に優れるデバイスの製造に用いることができる、ヘテロ接合を含むバッファ層を有する窒化物半導体ウェハを提供することができる。   According to one embodiment of the present invention, a nitride semiconductor wafer having a buffer layer including a heterojunction, which can be used for manufacturing a device having excellent on-resistance characteristics and buffer leak characteristics can be provided.

図１（ａ）は、実施の形態に係る窒化物半導体ウェハの垂直断面図である。図１（ｂ）は、窒化物半導体ウェハを用いて形成されたＨＥＭＴの垂直断面図である。FIG. 1A is a vertical sectional view of a nitride semiconductor wafer according to an embodiment. FIG. 1B is a vertical sectional view of a HEMT formed using a nitride semiconductor wafer. 図２は、本実施の形態のＨＥＭＴ及び比較例のＨＥＭＴにおける、ゲート電圧とバッファリーク電流の関係を表すグラフである。FIG. 2 is a graph showing the relationship between the gate voltage and the buffer leakage current in the HEMT of this embodiment and the HEMT of the comparative example. 図３は、本実施の形態のＨＥＭＴにおける、ＡｌｙＧａ１−ｙＮ層のＣのシート濃度Ｎｓとバッファリーク電流との関係を表すグラフである。FIG. 3 is a graph showing the relationship between the C sheet concentration Ns of the Al y Ga 1-y N layer and the buffer leakage current in the HEMT of this embodiment. 図４は、本実施の形態のＨＥＭＴにおける、ＡｌｙＧａ１−ｙＮ層のＣのシート濃度Ｎｓと飽和ドレイン電流との関係を表すグラフである。FIG. 4 is a graph showing the relationship between the sheet concentration Ns of C in the Al y Ga 1-y N layer and the saturation drain current in the HEMT of this embodiment.

（実施の形態の要点）
本発明の一実施の形態は、基板と、ＡｌｘＧａ１−ｘＮ層（０≦ｘ≦０．０５）と、ＡｌｙＧａ１−ｙＮ層（０＜ｙ≦１、かつｘ＜ｙ）との交互層を含む、基板上のバッファ層と、を有し、交互層において、ＡｌｙＧａ１−ｙＮ層のみがアクセプタを含む、窒化物半導体ウェハを提供する。 (Main points of the embodiment)
An embodiment of the present invention includes a substrate, an Al x Ga 1-x N layer (0 ≦ x ≦ 0.05), an Al y Ga 1-y N layer (0 <y ≦ 1, and x <y And a buffer layer on the substrate, wherein only the Al y Ga 1-y N layer includes an acceptor.

窒化物半導体ウェハを用いてＨＥＭＴ等のデバイスを製造した場合、交互層において、ＡｌｘＧａ１−ｘＮ層がアクセプタを含まず、ＡｌｙＧａ１−ｙＮ層のみがアクセプタを含むことにより、ＡｌｘＧａ１−ｘＮ層とＡｌｙＧａ１−ｙＮ層とのヘテロ界面に生じる分極電荷に起因する誘起電荷を効果的に抑制することができる。さらに、ＡｌｙＧａ１−ｙＮ層のアクセプタ濃度を適切な値に設定することにより、アクセプタによるチャネル層の自由電子の捕獲を最小限に抑えてキャリア伝導の低減を抑えつつ、バッファ層における自由キャリア発生を抑制して高い絶縁特性を確保することができる。 When a device such as HEMT is manufactured using a nitride semiconductor wafer, in the alternating layers, the Al x Ga 1-x N layer does not include an acceptor, and only the Al y Ga 1-y N layer includes an acceptor, It is possible to effectively suppress the induced charge caused by the polarization charge generated at the hetero interface between the Al x Ga 1-x N layer and the Al y Ga 1-y N layer. Furthermore, by setting the acceptor concentration of the Al y Ga 1-y N layer to an appropriate value, the free electron trapping in the channel layer by the acceptor is minimized, the reduction in carrier conduction is suppressed, and the freedom in the buffer layer is reduced. Carrier generation can be suppressed and high insulation characteristics can be ensured.

〔実施の形態〕
（窒化物半導体ウェハの構造）
図１（ａ）は、実施の形態に係る窒化物半導体ウェハの垂直断面図である。図１（ｂ）は、窒化物半導体ウェハを用いて形成されたＨＥＭＴ（高電子移動度トランジスタ）の垂直断面図である。 Embodiment
(Nitride semiconductor wafer structure)
FIG. 1A is a vertical sectional view of a nitride semiconductor wafer according to an embodiment. FIG. 1B is a vertical sectional view of a HEMT (High Electron Mobility Transistor) formed using a nitride semiconductor wafer.

窒化物半導体ウェハ１は、基板１１、基板１１上のＧａＮバッファ層１２、ＧａＮバッファ層１２上のバッファ層１３を有する。ＨＥＭＴ２は、窒化物半導体ウェハ１と、窒化物半導体ウェハ１のバッファ層１３上のチャネル層２１と、チャネル層２１上の障壁層２２と、障壁層２２上のソース電極２３、ドレイン電極２４、及びゲート電極２５を有する。   The nitride semiconductor wafer 1 has a substrate 11, a GaN buffer layer 12 on the substrate 11, and a buffer layer 13 on the GaN buffer layer 12. The HEMT 2 includes a nitride semiconductor wafer 1, a channel layer 21 on the buffer layer 13 of the nitride semiconductor wafer 1, a barrier layer 22 on the channel layer 21, a source electrode 23, a drain electrode 24 on the barrier layer 22, and A gate electrode 25 is provided.

基板１１は、例えば、ノンドープのＧａＮ基板、又はＦｅドープＧａＮ基板である。また、基板１１とＧａＮバッファ層１２との間に核発生層と呼ばれるＡｌＮ又はＡｌＧａＮからなる層を挿入することにより、半絶縁性ＳｉＣ基板やＳｉ基板等を基板１１として使用することができる。   The substrate 11 is, for example, a non-doped GaN substrate or a Fe-doped GaN substrate. Further, by inserting a layer made of AlN or AlGaN called a nucleation layer between the substrate 11 and the GaN buffer layer 12, a semi-insulating SiC substrate, Si substrate, or the like can be used as the substrate 11.

ＧａＮバッファ層１２は、例えば、ＧａＮ結晶からなる。チャネル層２１は、例えば、ＧａＮ結晶又はＩｎＧａＮ結晶からなり、２０ｎｍの厚さを有する。障壁層２２は、例えば、ＡｌＧａＮ結晶又はＩｎＡｌＮ結晶からなり、２５ｎｍの厚さを有する。ソース電極２３及びドレイン電極２４は、例えば、ＴｉとＡｌの積層体からなる。ゲート電極２５は、例えば、ＰｄとＡｕの積層体からなる。   The GaN buffer layer 12 is made of, for example, a GaN crystal. The channel layer 21 is made of, for example, a GaN crystal or an InGaN crystal and has a thickness of 20 nm. The barrier layer 22 is made of, for example, an AlGaN crystal or an InAlN crystal and has a thickness of 25 nm. The source electrode 23 and the drain electrode 24 are made of a laminate of Ti and Al, for example. The gate electrode 25 is made of, for example, a stacked body of Pd and Au.

バッファ層１３は、ＡｌｘＧａ１−ｘＮ層（０≦ｘ≦０．０５）１３ａと、ＡｌｙＧａ１−ｙＮ層（０＜ｙ≦１、かつｘ＜ｙ）１３ｂとの交互層、すなわちＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂとを交互に積層して形成される層からなる。ＡｌｘＧａ１−ｘＮ層１３ａ及びＡｌｙＧａ１−ｙＮ層１３ｂの各々の厚さは、例えば、５０ｎｍである。 The buffer layer 13 is composed of Al x Ga 1-x N layers (0 ≦ x ≦ 0.05) 13a and Al y Ga 1-y N layers (0 <y ≦ 1 and x <y) 13b. That is, it is composed of a layer formed by alternately laminating Al x Ga 1-x N layers 13a and Al y Ga 1-y N layers 13b. The thickness of each of the Al x Ga 1-x N layer 13a and the Al y Ga 1-y N layer 13b is, for example, 50 nm.

ＡｌｘＧａ１−ｘＮ層１３ａは、ＡｌｘＧａ１−ｘＮ結晶（０≦ｘ≦０．０５）からなり、アクセプタを実質的に含まない。ＡｌｙＧａ１−ｙＮ層１３ｂは、ＡｌｙＧａ１−ｙＮ結晶（０＜ｙ≦１、かつｘ＜ｙ）を母結晶とし、アクセプタを含む。 The Al x Ga 1-x N layer 13a is made of an Al x Ga 1-x N crystal (0 ≦ x ≦ 0.05) and does not substantially include an acceptor. The Al y Ga 1-y N layer 13b uses an Al y Ga 1-y N crystal (0 <y ≦ 1 and x <y) as a mother crystal and includes an acceptor.

すなわち、このＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂの交互層において、ＡｌｙＧａ１−ｙＮ層１３ｂのみが実質的にアクセプタを含む。ＡｌｘＧａ１−ｘＮ層１３ａには意図的にアクセプタがドーピングされないため、ＡｌｙＧａ１−ｙＮ層１３ｂから拡散移動したようなアクセプタがＡｌｘＧａ１−ｘＮ層１３ａ中に存在したとしても、その濃度は極めて低く、電気的測定の検出下限以下（例えば、８×１０１６（ｃｍ−３）以下）である。 That is, in the alternating layers of the Al x Ga 1-x N layer 13a and the Al y Ga 1-y N layer 13b, only the Al y Ga 1-y N layer 13b substantially includes an acceptor. Since Al x Ga intentionally acceptor in 1-x N layer 13a is not doped, acceptor such as diffused moved from Al y Ga 1-y N layer 13b is present in the Al x Ga 1-x N layer 13a However, the concentration is extremely low, and is below the lower limit of detection of electrical measurement (for example, 8 × 10 16 (cm −3 ) or less).

仮に、ＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂの両方がアクセプタを含むとすると、ＡｌｘＧａ１−ｘＮ層１３ａ中のアクセプタ準位がフェルミ準位をピン止めして、ＡｌｙＧａ１−ｙＮ層１３ｂにアクセプタをドーピングする効果が弱まる。このため、分極電荷に起因する誘起電荷の補償が不十分となり、バッファ層１３の絶縁特性が低減するおそれがある。また、逆に、バッファ層１３の絶縁特性が確保されるような濃度でアクセプタをドーピングすると、チャネル層の自由電子も捕獲してしまい、デバイスのオン抵抗が上昇するおそれが高い。 If both the Al x Ga 1-x N layer 13a and the Al y Ga 1-y N layer 13b include an acceptor, the acceptor level in the Al x Ga 1-x N layer 13a pins the Fermi level. The effect of doping the acceptor into the Al y Ga 1-y N layer 13b is weakened. For this reason, compensation of the induced charge due to the polarization charge becomes insufficient, and there is a possibility that the insulating characteristics of the buffer layer 13 are reduced. Conversely, if the acceptor is doped at such a concentration that the insulating characteristics of the buffer layer 13 are ensured, free electrons in the channel layer are also captured, and the on-resistance of the device is likely to increase.

なお、ＡｌｘＧａ１−ｘＮ層１３ａの分極電荷密度は小さい方が好ましいので、ＡｌｘＧａ１−ｘＮ層１３ａのＡｌ組成ｘは、ｘ＝０であることが好ましい。 Since polarization density of the Al x Ga 1-x N layer 13a is preferably small, Al composition x of Al x Ga 1-x N layer 13a is preferably x = 0.

また、ＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂの層数は、図１（ａ）、（ｂ）に示されるものに限られない。また、バッファ層１３において、ＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂの層数は、同じであってもよいし、一方が他方より１つ多くてもよい。 Moreover, the number of layers of the Al x Ga 1-x N layer 13a and the Al y Ga 1-y N layer 13b is not limited to that shown in FIGS. Moreover, in the buffer layer 13, the number of layers of the Al x Ga 1-x N layer 13a and the Al y Ga 1-y N layer 13b may be the same, or one may be one more than the other.

ＡｌｙＧａ１−ｙＮ層１３ｂに含まれるアクセプタとして、ベリリウム（Ｂｅ）、マグネシウム（Ｍｇ）、カルシウム（Ｃａ）、亜鉛（Ｚｎ）、又はカドミウム（Ｃｄ）等のII族金属、鉄（Ｆｅ）又はマンガン（Ｍｎ）等の遷移金属、炭素（Ｃ）等の両性不純物を用いることができる。中でも、本実施の形態においては、ドーピングの容易さの観点から、Ｆｅ、Ｚｎ、又はＣがアクセプタとして好ましく、Ｃが特に好ましい。 As an acceptor included in the Al y Ga 1-y N layer 13b, a group II metal such as beryllium (Be), magnesium (Mg), calcium (Ca), zinc (Zn), or cadmium (Cd), iron (Fe) Alternatively, transition metals such as manganese (Mn) and amphoteric impurities such as carbon (C) can be used. Among these, in the present embodiment, Fe, Zn, or C is preferable as an acceptor from the viewpoint of easy doping, and C is particularly preferable.

バッファ層１３におけるＡｌｙＧａ１−ｙＮ層１３ｂのうちの少なくとも最上層のＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙは、０＜ｙ≦０．４であり、かつアクセプタのシート濃度Ｎｓは、５×１０１３・ｙ＜Ｎｓ≦２×１０１３（ｃｍ−２）であることが好ましい。 Al composition y of at least the uppermost layer of Al y Ga 1-y N layer 13b of the Al y Ga 1-y N layer 13b in the buffer layer 13, 0 <y ≦ 0.4, and the sheet density of the acceptor Ns is preferably 5 × 10 13 · y <Ns ≦ 2 × 10 13 (cm −2 ).

最上層のＡｌｙＧａ１−ｙＮ層１３ｂのアクセプタのシート濃度Ｎｓが５×１０１３・ｙ以下の場合、ＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂとのヘテロ界面に生じる分極電荷に起因する誘起電荷を十分に補償することができず、バッファリーク電流を十分に抑えることができないおそれがある。一方、アクセプタのシート濃度Ｎｓが２×１０１３（ｃｍ−２）より高い場合、アクセプタがチャネル層２１の自由電子を捕獲し、ＨＥＭＴ２のオン抵抗が上昇するおそれがある。 When the sheet concentration Ns of the acceptor of the uppermost Al y Ga 1-y N layer 13b is 5 × 10 13 · y or less, the Al x Ga 1-x N layer 13a and the Al y Ga 1-y N layer 13b There is a possibility that the induced charge caused by the polarization charge generated at the heterointerface cannot be sufficiently compensated and the buffer leakage current cannot be sufficiently suppressed. On the other hand, when the sheet concentration Ns of the acceptor is higher than 2 × 10 13 (cm −2 ), the acceptor may capture free electrons of the channel layer 21 and increase the on-resistance of the HEMT 2.

また、最上層のＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙが０．４よりも大きい場合、分極電荷に起因する誘起電荷の量が多くなりすぎ、これを補償するためにシート濃度Ｎｓが２×１０１３（ｃｍ−２）より高いアクセプタが必要になるおそれがある。 Further, when the Al composition y of the uppermost Al y Ga 1-y N layer 13b is larger than 0.4, the amount of induced charges due to the polarization charge becomes too large, and the sheet concentration Ns is compensated for this. May require an acceptor higher than 2 × 10 13 (cm −2 ).

上から２層目以下のＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙ及び一層あたりのアクセプタのシート濃度Ｎｓは、最上層のＡｌｙＧａ１−ｙＮ層１３ｂのものと異なっていてもよいが、チャネル層２１の自由電子濃度への影響を極力抑えつつバッファリーク電流を極力抑えるために、最上層と同様の条件「Ａｌ組成ｙが０＜ｙ≦０．４であり、かつアクセプタのシート濃度Ｎｓが５×１０１３・ｙ＜Ｎｓ≦２×１０１３（ｃｍ−２）」を満たすことが好ましい。また、製造が容易になるため、全てのＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙ及び一層あたりのアクセプタのシート濃度Ｎｓが等しいことがより好ましい。 The Al composition y of the second and lower layers of the Al y Ga 1-y N layer 13b and the acceptor sheet concentration Ns per layer may be different from those of the uppermost layer of the Al y Ga 1-y N layer 13b. However, in order to suppress the buffer leakage current as much as possible while suppressing the influence on the free electron concentration of the channel layer 21 as much as possible, the condition “Al composition y is 0 <y ≦ 0.4 and the acceptor It is preferable that the sheet concentration Ns satisfies 5 × 10 13 · y <Ns ≦ 2 × 10 13 (cm −2 ) ”. Further, since the production is facilitated, and it is more preferable that all of Al y Ga 1-y N layer 13b of Al composition y and sheet density Ns of the acceptor per layer is equal.

（実施の形態の効果）
本実施の形態によれば、バッファ層１３において、ＡｌｘＧａ１−ｘＮ層１３ａがアクセプタを含まず、ＡｌｙＧａ１−ｙＮ層１３ｂのみがアクセプタを含むことにより、ＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂとのヘテロ界面に生じる分極電荷に起因する誘起電荷を効果的に抑制することができる。さらに、ＡｌｙＧａ１−ｙＮ層１３ｂのアクセプタ濃度を本実施の形態に係る適切な値に設定することにより、アクセプタによるチャネル層２１の自由電子の捕獲を最小限に抑えてキャリア伝導の低減を抑えつつ、バッファ層１３における自由キャリア発生を抑制して高い絶縁特性を確保することができる。 (Effect of embodiment)
According to the present embodiment, in the buffer layer 13, the Al x Ga 1-x N layer 13a does not include an acceptor, and only the Al y Ga 1-y N layer 13b includes an acceptor, whereby the Al x Ga 1- The induced charge resulting from the polarization charge generated at the heterointerface between the xN layer 13a and the Al y Ga 1-y N layer 13b can be effectively suppressed. Furthermore, by setting the acceptor concentration of the Al y Ga 1-y N layer 13b to an appropriate value according to the present embodiment, the trapping of free electrons in the channel layer 21 by the acceptor is minimized, and the carrier conduction is reduced. While suppressing the above, generation of free carriers in the buffer layer 13 can be suppressed to ensure high insulation characteristics.

したがって、本実施の形態の窒化物半導体ウェハ１を用いて、オン抵抗特性及びバッファリーク特性に優れるＨＥＭＴ２等のデバイスを製造することができる。   Therefore, a device such as HEMT 2 having excellent on-resistance characteristics and buffer leak characteristics can be manufactured using the nitride semiconductor wafer 1 of the present embodiment.

〔他の実施の形態〕
以上、本発明の実施の形態を説明したが、本発明は、上記実施の形態に限定されず、発明の主旨を逸脱しない範囲内において種々変形実施が可能である。 [Other Embodiments]
Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the invention.

例えば、バッファ層１３において、ＡｌｘＧａ１−ｘＮ層１３ａ及びＡｌｙＧａ１−ｙＮ層１３ｂの代わりに、ＩｎＡｌＮ結晶からなる層を用いることも可能であるが、発生する分極電荷密度や、上記実施の形態と同様の効果を得るためのアクセプタ濃度はＡｌｘＧａ１−ｘＮ層１３ａ及びＡｌｙＧａ１−ｙＮ層１３ｂと異なる可能性がある。 For example, in the buffer layer 13, a layer made of InAlN crystal can be used instead of the Al x Ga 1-x N layer 13a and the Al y Ga 1-y N layer 13b. The acceptor concentration for obtaining the same effect as in the above embodiment may be different from that of the Al x Ga 1-x N layer 13a and the Al y Ga 1-y N layer 13b.

上記実施の形態に係るＨＥＭＴ２を製造し、各種評価を行った。以下に、評価の方法及び結果を述べる。   The HEMT 2 according to the above embodiment was manufactured and subjected to various evaluations. The evaluation method and results will be described below.

（バッファ層の製造）
本実施例において用いたバッファ層１３の製造方法について述べる。 (Manufacture of buffer layer)
A method for manufacturing the buffer layer 13 used in this example will be described.

有機金属気相成膜法によりＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂを形成した。後述する各種の評価のために、ＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙ及びアクセプタ濃度の異なる複数のＨＥＭＴ２を形成した。なお、ＡｌｘＧａ１−ｘＮ層１３ａのＡｌ組成ｘは０に固定した。 An Al x Ga 1-x N layer 13a and an Al y Ga 1-y N layer 13b were formed by a metal organic vapor deposition method. For various evaluations to be described later, a plurality of HEMTs 2 having different Al compositions y and acceptor concentrations of the Al y Ga 1-y N layer 13b were formed. The Al composition x of the Al x Ga 1-x N layer 13a was fixed at 0.

Ｇａ原料としてトリメチルガリウム又はトリエチルガリウムを用いた。Ａｌ原料としてトリメチルアルミニウムを用いた。窒素原料としてアンモニアを用い、一部、ヒドラジン系材料を用いた。また、アクセプタとしてのＣの原料として、III族原料又はＣＢｒ４を用いた。アクセプタとしてのＦｅの原料としてＦｅＣｌ３を用いた。アクセプタとしてのＺｎの原料としてジメチルＺｎを用いた。 Trimethyl gallium or triethyl gallium was used as a Ga raw material. Trimethylaluminum was used as the Al raw material. Ammonia was used as a nitrogen raw material, and a hydrazine-based material was partially used. Further, a group III material or CBr 4 was used as a material for C as an acceptor. FeCl 3 was used as a raw material for Fe as an acceptor. Dimethyl Zn was used as a raw material for Zn as an acceptor.

Ｖ／III比（Ｖ族原料の濃度とIII族原料の濃度の比）、成膜温度（サセプタ温度）、成長圧力を適宜変えてＡｌＧａＮ結晶を成長させながら間欠的にＣをドーピングし、ＡｌｘＧａ１−ｘＮ層１３ａとＡｌｙＧａ１−ｙＮ層１３ｂを形成した。そして、元素分析により所望のＣ濃度となったものを選別して後述する評価を行った。ここで、Ｖ／III比は概ね１０〜１００，０００の範囲で調整した。成膜温度は概ね８００〜１，２５０℃の範囲で調整した。成長圧力は概ね１０ｋ〜１００ｋＰａの範囲で調整した。キャリアガスとしては、水素、窒素、又はそれらの混合気体を用い、混合気体の混合割合は随時調整した。 Al x is doped intermittently while growing an AlGaN crystal by appropriately changing the V / III ratio (ratio of the concentration of the group V material to the concentration of the group III material), the film formation temperature (susceptor temperature), and the growth pressure. A Ga 1-x N layer 13a and an Al y Ga 1-y N layer 13b were formed. And the thing which became the desired C density | concentration by the elemental analysis was selected, and evaluation mentioned later was performed. Here, the V / III ratio was adjusted in the range of approximately 10 to 100,000. The film forming temperature was generally adjusted in the range of 800 to 1,250 ° C. The growth pressure was generally adjusted in the range of 10 to 100 kPa. As the carrier gas, hydrogen, nitrogen, or a mixed gas thereof was used, and the mixing ratio of the mixed gas was adjusted as needed.

なお、成膜条件を変えても、後述するＨＥＭＴ２についての評価結果に変化はなかった。ＨＥＭＴ２の特性はＡｌ組成やＣ等のアクセプタの濃度で一義的に決まっており、成膜条件にはあまり依存しないものと考えられる。   In addition, even if the film forming conditions were changed, there was no change in the evaluation results for HEMT2, which will be described later. The characteristics of HEMT2 are uniquely determined by the acceptor concentration such as the Al composition and C, and are considered to be less dependent on the film forming conditions.

また、基板１１として、ノンドープのＧａＮ基板とＦｅドープＧａＮ基板を用いたが、いずれも評価結果に変化はなかった。また、チャネル層２１と障壁層２２に、ＧａＮ層とＡｌＧａＮ層の組み合わせと、ＩｎＧａＮ層とＩｎＡｌＮ層の組み合わせを用いたが、いずれも評価結果に変化はなかった。   Moreover, although the non-doped GaN substrate and the Fe-doped GaN substrate were used as the substrate 11, the evaluation results did not change in any case. In addition, the channel layer 21 and the barrier layer 22 were a combination of a GaN layer and an AlGaN layer, and a combination of an InGaN layer and an InAlN layer.

（バッファ層の絶縁特性の評価）
図２は、本実施の形態のＨＥＭＴ２及び比較例のＨＥＭＴにおける、ゲート電圧とバッファリーク電流の関係を表すグラフである。図２の横軸はゲート電圧（Ｖ）を表し、縦軸はバッファリーク電流であるドレイン電流（Ａ）を表す。線αは本実施の形態のＨＥＭＴ２の測定結果を表し、線βは比較例のＨＥＭＴの測定結果を表す。 (Evaluation of insulation properties of buffer layer)
FIG. 2 is a graph showing the relationship between the gate voltage and the buffer leakage current in the HEMT 2 of the present embodiment and the HEMT of the comparative example. The horizontal axis in FIG. 2 represents the gate voltage (V), and the vertical axis represents the drain current (A) that is a buffer leakage current. Line α represents the measurement result of HEMT2 of the present embodiment, and line β represents the measurement result of HEMT of the comparative example.

本測定に用いたＨＥＭＴ２におけるバッファ層１３は、厚さ５０ｎｍの２層のＡｌｘＧａ１−ｘＮ層１３ａと、厚さ５０ｎｍの３層のＡｌｙＧａ１−ｙＮ層１３ｂから構成され、ＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙとアクセプタとしてのＣのシート濃度Ｎｓは、それぞれ０．１５と、２×１０１３（ｃｍ−２）である。ＡｌｘＧａ１−ｘＮ層１３ａには、Ｃを意図的にドーピングしておらず、Ｃの濃度は電気的測定の検出下限以下（８×１０１６（ｃｍ−３）以下）であると推測される。 The buffer layer 13 in the HEMT 2 used for this measurement is composed of two Al x Ga 1-x N layers 13a having a thickness of 50 nm and three Al y Ga 1-y N layers 13b having a thickness of 50 nm. The Al composition y of the Al y Ga 1-y N layer 13b and the sheet concentration Ns of C as an acceptor are 0.15 and 2 × 10 13 (cm −2 ), respectively. The Al x Ga 1-x N layer 13a is not intentionally doped with C, and the concentration of C is estimated to be lower than the lower detection limit of electrical measurement (8 × 10 16 (cm −3 ) or lower). Is done.

また、比較例のＨＥＭＴは、本測定に用いられるＨＥＭＴ２のＡｌｘＧａ１−ｘＮ層１３ａの代わりに厚さ５０ｎｍのアクセプタを含まないＧａＮ層が形成され、ＡｌｙＧａ１−ｙＮ層１３ｂの代わりに厚さ５０ｎｍのアクセプタとしてのＣを含むＧａＮ層が形成されたトランジスタである。このＣを含むＧａＮ層の１層あたりのＣのシート濃度Ｎｓは、２×１０１３（ｃｍ−２）である。 In addition, in the HEMT of the comparative example, a GaN layer not including an acceptor having a thickness of 50 nm is formed instead of the Al x Ga 1-x N layer 13a of the HEMT 2 used in this measurement, and the Al y Ga 1-y N layer 13b is formed. In this transistor, a GaN layer containing C as an acceptor having a thickness of 50 nm is formed. The sheet concentration Ns of C per GaN layer containing C is 2 × 10 13 (cm −2 ).

また、本実施の形態のＨＥＭＴ２及び比較例のＨＥＭＴのゲート長は２μｍであり、ゲート幅は２０μｍである。測定は、ドレイン電圧を１００Ｖとして行った。   The gate length of the HEMT 2 of this embodiment and the HEMT of the comparative example is 2 μm, and the gate width is 20 μm. The measurement was performed with a drain voltage of 100V.

図２によれば、比較例のＨＥＭＴのピンチオフ時のバッファリーク電流がおよそ１×１０−５（Ａ）であるのに対して、実施の形態のＨＥＭＴ２のピンチオフ時のバッファリーク電流は１×１０−５（Ａ）以下であり、比較例よりも２桁以上小さい。 According to FIG. 2, the buffer leakage current at the time of pinch-off of the HEMT of the comparative example is about 1 × 10 −5 (A), whereas the buffer leakage current at the time of pinch-off of the HEMT 2 of the embodiment is 1 × 10 5. -5 (A) or less, two orders of magnitude or more smaller than the comparative example.

２×１０１３（ｃｍ−２）という比較的低いシート濃度ＮｓのＣにより、リーク電流がこれほど抑えられる理由については物理的に明らかではないが、本実施の形態のバッファ層１３においては、ＡｌｙＧａ１−ｙＮ層１３ｂの一方の面において生じた分極電荷を、反対の面において生じた反対極性の分極電荷がある程度相殺するためであると考えられる。 Although the reason why the leakage current is suppressed so much by C having a relatively low sheet concentration Ns of 2 × 10 13 (cm −2 ) is not clear, in the buffer layer 13 of the present embodiment, Al This is probably because the polarization charge generated on one surface of the y Ga 1-y N layer 13b cancels to some extent the polarization charge of the opposite polarity generated on the other surface.

（絶縁特性のアクセプタ濃度依存性の評価）
図３は、ＨＥＭＴ２に−８Ｖのゲート電圧（Ｖｇ）を印加してピンチオフさせた状態における、ＡｌｙＧａ１−ｙＮ層１３ｂのアクセプタとしてのＣのシート濃度Ｎｓとバッファリーク電流であるドレイン電流との関係を表すグラフである。図３の横軸はＡｌｙＧａ１−ｙＮ層１３ｂのＣのシート濃度Ｎｓ（ｃｍ−２）を表し、縦軸はドレイン電流（Ａ）を表す。図３は、ＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙがそれぞれ０．１、０．２、０．３である３種のＨＥＭＴ２についての測定結果を示す。なお、ＡｌｙＧａ１−ｙＮ層１３ｂ以外の部材の構成はすべてのＨＥＭＴ２において同じである。 (Evaluation of acceptor concentration dependence of insulation properties)
FIG. 3 shows the sheet concentration Ns of C as an acceptor of the Al y Ga 1-y N layer 13b and the drain current which is a buffer leak current in a state where the gate voltage (Vg) of −8V is applied to the HEMT 2 and pinched off. It is a graph showing the relationship. The horizontal axis in FIG. 3 represents the C sheet concentration Ns (cm −2 ) of the Al y Ga 1-y N layer 13b, and the vertical axis represents the drain current (A). FIG. 3 shows measurement results for three types of HEMTs 2 in which the Al composition y of the Al y Ga 1-y N layer 13b is 0.1, 0.2, and 0.3, respectively. The configuration of members other than the Al y Ga 1-y N layer 13b is the same in all HEMTs 2.

図３は、Ｃのシート濃度Ｎｓがある境界値よりも高くなると、バッファリーク電流がさらに大きく低減し、１×１０−９（Ａ）程度になること示している。ＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙが大きいほど、この境界値は大きくなり、ｙ＝０．１、０．２、０．３のときの境界値は、それぞれおよそ５×１０１２（ｃｍ−２）、１×１０１３（ｃｍ−２）、１．５×１０１３（ｃｍ−２）である。 FIG. 3 shows that when the sheet density Ns of C becomes higher than a certain boundary value, the buffer leakage current is further greatly reduced to about 1 × 10 −9 (A). The larger the Al composition y of the Al y Ga 1-y N layer 13b, the larger this boundary value, and the boundary values when y = 0.1, 0.2, and 0.3 are about 5 × 10 12 respectively. (Cm −2 ), 1 × 10 13 (cm −2 ), and 1.5 × 10 13 (cm −2 ).

この境界値の値は、前述の“O. Ambacherら，Journal of Applied Physics，Vol.87，334頁，2000”に示される関係式「ＡｌｘＧａ１−ｘＮ層のＡｌ組成ｘがｘ≦０．６のときは、ＡｌＧａＮ層とＧａＮ層の界面の分極電荷密度ＮｐはＮｐ＝５×１０１３・ｘ（ｃｍ−２）、ｘ＞０．６のときは、Ｎｐ＝１×１０１４・ｘ−３×１０１３（ｃｍ−２）」から算出されるＡｌｙＧａ１−ｙＮ層１３ｂとＡｌｘＧａ１−ｘＮ層１３ａ（ｘが０．０５以下と小さいため、近似的にＧａＮ層として扱うことができる）の界面の分極電荷密度Ｎｐにほぼ等しい。これは、分極電荷に起因する誘起電荷を補償するために、分極電荷密度Ｎｐと同等のシート濃度のアクセプタが必要になることによると考えられる。 The value of this boundary value is expressed by the relational expression “Al x Ga 1-x N layer x of the Al x Ga 1-x N layer x ≦ x ≦ O. Ambacher et al., Journal of Applied Physics, Vol. 87, page 334, 2000”. When 0.6, the polarization charge density Np at the interface between the AlGaN layer and the GaN layer is Np = 5 × 10 13 · x (cm −2 ), and when x> 0.6, Np = 1 × 10 14. The Al y Ga 1-y N layer 13b and the Al x Ga 1-x N layer 13a calculated from “x−3 × 10 13 (cm −2 )” (since x is as small as 0.05 or less, approximately The polarization charge density Np at the interface of the interface (which can be treated as a layer). This is considered to be because an acceptor having a sheet density equivalent to the polarization charge density Np is required to compensate for the induced charge caused by the polarization charge.

したがって、ＡｌｙＧａ１−ｙＮ層１３ｂのアクセプタのシート濃度ＮｓがＡｌｙＧａ１−ｙＮ層１３ｂの分極電荷密度Ｎｐよりも高いとき、すなわちＡｌ組成ｙがｙ≦０．６のときは５×１０１３・ｙ（ｃｍ−２）よりも高く、ｙ＞０．６のときは１×１０１４・ｙ−３×１０１３（ｃｍ−２）よりも高いときに、高いバッファリーク電流の抑制効果が得られるといえる。 Therefore, when the Al y Ga 1-y N layer 13b sheet concentration Ns of acceptors is higher than the polarization density Np of Al y Ga 1-y N layer 13b, that is, when the Al composition y is y ≦ 0.6 is High buffer leakage current when higher than 5 × 10 13 · y (cm −2 ), and higher than 1 × 10 14 · y−3 × 10 13 (cm −2 ) when y> 0.6 It can be said that a suppression effect is obtained.

アクセプタのシート濃度Ｎｓが高いほどバッファリーク電流が小さくなるのは、アクセプタのシート濃度Ｎｓが高い方がバッファ層１３のフェルミ準位の位置が安定するためと考えられる。これはまた、導入したアクセプタが、単にバッファ層１３中の分極電荷による誘起電荷を補償するだけでなく、ＡｌｘＧａ１−ｘＮ層１３ａ中、あるいはＡｌｙＧａ１−ｙＮ層１３ｂ中に含まれる残留ドナー型不純物も補償しているためと思われる。一般に、バッファ層の残留ドナー型不純物のシート濃度は、チャネル層の自由電子のシート濃度（およそ１〜２×１０１３（ｃｍ−２））に影響を及ぼさない程度に制御され、それはチャネル層のシート濃度の悪くても半分以下、つまりおよそ１×１０１３（ｃｍ−２）以下である。 The reason why the buffer leak current decreases as the acceptor sheet concentration Ns increases is that the higher the acceptor sheet concentration Ns, the more stable the Fermi level position of the buffer layer 13 is. This is also because the introduced acceptor not only compensates the induced charge due to the polarization charge in the buffer layer 13, but also in the Al x Ga 1-x N layer 13a or the Al y Ga 1-y N layer 13b. This is probably because the residual donor-type impurities contained are also compensated. In general, the sheet concentration of residual donor-type impurities in the buffer layer is controlled so as not to affect the sheet concentration of free electrons in the channel layer (approximately 1 to 2 × 10 13 (cm −2 )). Even if the sheet density is low, it is less than half, that is, approximately 1 × 10 13 (cm −2 ) or less.

（キャリア伝導特性のアクセプタ濃度依存性の評価）
図４は、本実施の形態のＨＥＭＴ２における、アクセプタとしてのＣのシート濃度Ｎｓと飽和ドレイン電流との関係を表すグラフである。図４の横軸はＡｌｙＧａ１−ｙＮ層１３ｂに含まれる１層あたりのＣのシート濃度Ｎｓ（ｃｍ−２）を表し、縦軸は飽和ドレイン電流（Ａ／ｍｍ）を表す。図４は、ＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙがそれぞれ０．１、０．２、０．３である３種のＨＥＭＴ２についての測定結果を示す。なお、ＡｌｙＧａ１−ｙＮ層１３ｂ以外の部材の構成はすべてのＨＥＭＴ２において同じである。 (Evaluation of acceptor concentration dependence of carrier conduction characteristics)
FIG. 4 is a graph showing the relationship between the sheet concentration Ns of C as an acceptor and the saturation drain current in the HEMT 2 of the present embodiment. The horizontal axis in FIG. 4 represents the sheet concentration Ns (cm −2 ) of C per layer included in the Al y Ga 1-y N layer 13b, and the vertical axis represents the saturated drain current (A / mm). FIG. 4 shows measurement results for three types of HEMTs 2 in which the Al composition y of the Al y Ga 1-y N layer 13b is 0.1, 0.2, and 0.3, respectively. The configuration of members other than the Al y Ga 1-y N layer 13b is the same in all HEMTs 2.

図４は、アクセプタであるＣのシート濃度Ｎｓが２×１０１３（ｃｍ−２）を超えると、飽和ドレイン電流が急激に低下することを示している。これは、チャネル層２１の自由電子のシート濃度に対するＡｌｙＧａ１−ｙＮ層１３ｂのアクセプタのシート濃度Ｎｓが高過ぎるため、チャネル層の自由電子がアクセプタに捕獲され、急激にフェルミ準位の位置がシフトすることによると考えられる。このため、ＡｌｙＧａ１−ｙＮ層１３ｂの一層あたりのアクセプタのシート濃度Ｎｓは、２×１０１３（ｃｍ−２）以下であることが好ましい。 FIG. 4 shows that when the sheet concentration Ns of the acceptor C exceeds 2 × 10 13 (cm −2 ), the saturation drain current rapidly decreases. This is because the acceptor sheet concentration Ns of the Al y Ga 1-y N layer 13b with respect to the free electron sheet concentration of the channel layer 21 is too high, so that the free electrons of the channel layer are trapped by the acceptor, and the Fermi level rapidly increases. This is thought to be due to the shift in position. Therefore, the sheet concentration Ns of the acceptor per layer of Al y Ga 1-y N layer 13b is preferably at 2 × 10 13 (cm -2) or less.

なお、ＨＥＭＴを含む電界効果トランジスタ（ＦＥＴ）系デバイスでは、多くの場合、チャネル層の自由電子のシート濃度はいずれも１×１０１３〜２×１０１３（ｃｍ−２）程度と近い値であるから、窒化物半導体ウェハ１を用いてＨＥＭＴ２以外のＦＥＴ系デバイスを形成する場合においても、ＡｌｙＧａ１−ｙＮ層１３ｂの一層あたりのアクセプタのシート濃度Ｎｓは、２×１０１３（ｃｍ−２）以下であることが好ましいといえる。 In many cases, in the field effect transistor (FET) -based device including HEMT, the free electron sheet concentration of the channel layer is close to about 1 × 10 13 to 2 × 10 13 (cm −2 ). from, in the case of forming a FET-based devices other than HEMT2 using nitride semiconductor wafer 1 is also, Al y Ga 1-y N layer sheet concentration Ns of the acceptor per layer of 13b is, 2 × 10 13 (cm - 2 ) It can be said that the following is preferable.

ここで、上述の高いバッファリーク電流の抑制効果を得るための条件「ｙ≦０．６のときはＮｓ＞５×１０１３・ｙ（ｃｍ−２）、ｙ＞０．６のときはＮｓ＞１×１０１４・ｙ−３×１０１３（ｃｍ−２）」に、チャネル層２１のキャリア伝導を妨げないための条件「Ｎｓ≦２×１０１３（ｃｍ−２）」を加えると、前の条件においてｙ＞０．４ではＮｓ＞２×１０１３（ｃｍ−２）となってしまうため、「０＜ｙ≦０．４、かつ５×１０１３・ｙ＜Ｎｓ≦２×１０１３（ｃｍ−２）」となる。 Here, the condition for obtaining the above-described effect of suppressing the high buffer leakage current is “Ns> 5 × 10 13 · y (cm −2 ) when y ≦ 0.6, and Ns> when y> 0.6. When the condition “Ns ≦ 2 × 10 13 (cm −2 )” for preventing the carrier conduction of the channel layer 21 is added to “1 × 10 14 · y−3 × 10 13 (cm −2 )”, Under the condition, if y> 0.4, Ns> 2 × 10 13 (cm −2 ), so “0 <y ≦ 0.4 and 5 × 10 13 · y <Ns ≦ 2 × 10 13 (cm -2 ) ".

したがって、チャネル層２１のキャリア伝導を妨げず、かつバッファリーク電流を効果的に抑えるためには、ＡｌｙＧａ１−ｙＮ層１３ｂが「０＜ｙ≦０．４、かつ５×１０１３・ｙ＜Ｎｓ≦２×１０１３（ｃｍ−２）」の条件を満たすことが好ましいといえる。 Therefore, in order not to hinder the carrier conduction of the channel layer 21 and to suppress the buffer leakage current effectively, the Al y Ga 1-y N layer 13b has a relationship of “0 <y ≦ 0.4 and 5 × 10 13. It can be said that it is preferable to satisfy the condition of “y <Ns ≦ 2 × 10 13 (cm −2 )”.

なお、Ｃの代わりにＦｅ、ＺｎをＡｌｙＧａ１−ｙＮ層１３ｂにドーピングして同様の実験を行ったところ、同様の測定結果が得られ、同様の条件によりチャネル層２１のキャリア伝導を妨げず、かつバッファリーク電流を効果的に抑えることができることがわかった。さらに、Ｃ、Ｆｅ、Ｚｎのうちの２種類又は３種類を組み合わせてＡｌｙＧａ１−ｙＮ層１３ｂにドーピングした場合も同様であった。 In addition, when the same experiment was conducted by doping Fe and Zn into the Al y Ga 1-y N layer 13b instead of C, the same measurement result was obtained, and the carrier conduction of the channel layer 21 was performed under the same conditions. It was found that the buffer leakage current can be effectively suppressed without hindering. Further, the same was true when the Al y Ga 1-y N layer 13b was doped by combining two or three of C, Fe, and Zn.

なお、本実施例においては、バッファ層１３中のすべてのＡｌｙＧａ１−ｙＮ層１３ｂのＡｌ組成ｙ及びＣのシート濃度Ｎｓを等しくしたが、少なくともチャネル層２１に最も近い最上層のＡｌｙＧａ１−ｙＮ層１３ｂが上記の条件「０＜ｙ≦０．４、かつ５×１０１３・ｙ＜Ｎｓ≦２×１０１３（ｃｍ−２）」を満たせば、同様の効果が得られる。 In this embodiment, the Al composition y of all the Al y Ga 1-y N layers 13 b and the sheet concentration Ns of C in the buffer layer 13 are made equal, but at least the uppermost Al layer closest to the channel layer 21 is used. The same effect can be obtained if the y Ga 1-y N layer 13b satisfies the above-mentioned conditions “0 <y ≦ 0.4 and 5 × 10 13 · y <Ns ≦ 2 × 10 13 (cm −2 )”. It is done.

上記に記載した実施の形態及び実施例は特許請求の範囲に係る発明を限定するものではない。また、実施の形態及び実施例の中で説明した特徴の組合せの全てが発明の課題を解決するための手段に必須であるとは限らない点に留意すべきである。   The embodiments and examples described above do not limit the invention according to the claims. It should be noted that not all combinations of features described in the embodiments and examples are necessarily essential to the means for solving the problems of the invention.

１ 窒化物半導体ウェハ
１１ 基板
１２ ＧａＮバッファ層
１３ バッファ層
１３ａ ＡｌｘＧａ１−ｘＮ層
１３ｂ ＡｌｙＧａ１−ｙＮ層 First nitride semiconductor wafer 11 substrate 12 GaN buffer layer 13 buffer layer 13a Al x Ga 1-x N layer 13b Al y Ga 1-y N layer

Claims (4)

基板と、
ＡｌｘＧａ１−ｘＮ層（０≦ｘ≦０．０５）と、ＡｌｙＧａ１−ｙＮ層（０＜ｙ≦１、かつｘ＜ｙ）との交互層を含む、前記基板上のバッファ層と、
を有し、
前記交互層において、前記ＡｌｙＧａ１−ｙＮ層のみがアクセプタを含み、
前記交互層の前記ＡｌｙＧａ１−ｙＮ層のうちの最上層のＡｌｙＧａ１−ｙＮ層のＡｌ組成ｙは、０＜ｙ≦０．４であり、
前記最上層のＡｌｙＧａ１−ｙＮ層の前記アクセプタのシート濃度Ｎｓは、５×１０１３・ｙ＜Ｎｓ≦２×１０１３（ｃｍ−２）である、
窒化物半導体ウェハ。 A substrate,
On the substrate, including alternating layers of Al x Ga 1-x N layers (0 ≦ x ≦ 0.05) and Al y Ga 1-y N layers (0 <y ≦ 1 and x <y) A buffer layer,
Have
In the alternating layers, only the Al y Ga 1-y N layer includes an acceptor,
It said Al y Ga 1-y N Al composition y of the uppermost Al y Ga 1-y N layer of layers of the alternating layer is 0 <y ≦ 0.4,
The acceptor sheet concentration Ns of the uppermost Al y Ga 1-y N layer is 5 × 10 13 · y <Ns ≦ 2 × 10 13 (cm −2 ).
Nitride semiconductor wafer. 前記アクセプタは、鉄（Ｆｅ）、炭素（Ｃ）、亜鉛（Ｚｎ）のうちの少なくとも１種類からなる、
請求項１に記載の窒化物半導体ウェハ。 The acceptor is composed of at least one of iron (Fe), carbon (C), and zinc (Zn).
The nitride semiconductor wafer according to claim 1 . 前記ＡｌｘＧａ１−ｘＮ層のＡｌ組成ｘは、ｘ＝０である、
請求項１又は２に記載の窒化物半導体ウェハ。 The Al composition x of the Al x Ga 1-x N layer is x = 0.
The nitride semiconductor wafer according to claim 1 or 2 . 前記交互層のすべての前記ＡｌｙＧａ１−ｙＮ層のＡｌ組成ｙ及び一層あたりの前記アクセプタのシート濃度Ｎｓが等しい、
請求項１〜３のいずれかに記載の窒化物半導体ウェハ。 The Al composition y of all the Al y Ga 1-y N layers of the alternating layers and the sheet concentration Ns of the acceptor per layer are equal,
The nitride semiconductor wafer according to any one of claims 1-3.
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